1) Field of the Invention
The present invention relates to a polarization scrambler, and an optical add/drop multiplexer, an optical route switching apparatus and a wavelength division multiplexing optical transmission system using the polarization scrambler.
2) Description of Related Art
It is general that the core of single mode fibers (SMF: Single Mode Fiber) used in optical transmission lines in optical transmission systems is not round but slightly elliptic, which induces birefringence. An optical signal inputted to such optical fiber is separated into two rectangular polarization mode components, that is, a fast-wave axis and a slow-wave axis.
Since the transmission speed in the fiber differs between the two polarization mode components separated into the fast-wave and the slow-wave, differential group delay (DGD: Differential Group Delay) generates between the modes. Generation of the differential group delay between the modes causes a difference in transmission speed between the fast-wave and the slow-wave even in one optical pulse, which may induce considerable degradation of the transmission quality, depending on the state of the polarization particularly in high-speed signals at short pulse intervals.
Polarization mode dispersion (PMD: Polarization Mode Dispersion) is a phenomenon that the signal waveform is distorted by that the cross sectional shape of an optical fiber is deviated from idealistic round by a small stress applied to the optical fiber. PMD has a characteristic that is fluctuated with time by a change in environments of the transmission line such as temperature, stress and the like, almost without correlation between the wavelengths.
A wavelength division multiplexing (WDM: Wavelength Division Multiplexing) optical transmission system having a transmission speed of 10 Gb/s per wave is put to practical use, at present. Further, practical application of an optical transmission system having a transmission speed of 40 Gb/s is under examination, at present. In such high-speed optical transmission systems, degradation of the transmission quality due to PDM as discussed above cannot be ignored, which is one of factors that limit the transmission distance.
Some modulation schemes for optically modulating data signals may require a device having relatively large polarization dependency of the transmission speed as a device constituting a receiver. In such case, necessity for a measure against the above-mentioned PMD increases in order to improve the receiving quality.
For the purpose of mitigating degradation of the transmission quality due to PMD, various methods are under examination. For example, there is a method for mitigating the maximum penalty due to PMD with a polarization scrambler in the transceiver, as disclosed in Patent Document 1 described below. Patent Documents 2 and 3 disclose a constitution where, in a point-to-point wavelength division multiplexing optical communication system having a pair of optical-transmitting and optical-receiving apparatuses opposite to each other, a polarization scrambler is disposed in the repeating stage on the transmission path.
Degradation of the transmission quality due to PMD changes from zero to the maximum according to the input polarization state to a medium having PMD, which leads to variation in degradation of the transmission quality. Thus, it is required to keep the transmission quality constant. However, the techniques disclosed in Patent Documents 1 through 3 apply a polarization scrambler to the input portion to scramble the input polarization state at any time, thereby to uniformly generate a state where the transmission quality degradation penalty due to PMD is small and a state where the same is large. As this, the transmission quality degradation stays neither in the best state nor the worst state so that the averaged transmission quality of the system is kept above a constant level.
In the techniques disclosed in Patent Documents 1 through 3, the speed of scrambling in the polarization scrambler is set to be sufficient to average the degradation of the transmission quality within one processing unit of error correction code.
As other known techniques relating to the present invention, there are techniques disclosed in Patent Document 4 and Non-Patent Document 1.
[Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2001-268010
[Patent Document 2] Japanese Patent Application Laid-Open Publication No. 2005-295559
[Patent Document 3] Japanese Patent Application Laid-Open Publication No. 2005-65273
[Patent Document 4] Japanese Patent Application Laid-Open Publication No. HEI 9-275378
[Non-Patent Document 1] Zhihong Li, Jinyu Mo, Yi Dong, Yixin Wang, Chao Lu, “Experimental evaluation of the effect of polarization scrambling speed on the performance of PMD mitigation using EFC,” OFC2004, MF69, 2004
However, when an optical add/drop multiplexer (OADM: Optical Add/Drop Multiplexer), particularly, a reconfigurable optical add/drop multiplexer (R-OADM: Reconfigurable Optical Add/Drop Multiplexer) or a wavelength-convertible dynamic optical add/drop multiplexer (D-OADM: Dynamic Optical Add/Drop Multiplexer), or an optical HUB device is applied on a transmission path, the transmission path is more complicated than a point-to-point transmission path that the techniques disclosed in Patent Documents 1 to 3 suppose. Thus, that what kind of signal is transmitted on what path changes according to circumstances.
The techniques disclosed in the above Patent Documents 1 to 3 set the scrambling speed to a speed sufficient to average degradation of the transmission quality within one processing unit for error correction code. However, high speed scrambling causes that the received waveform is fluctuated at high speed, which may cause an increase in noise in the receiving system. For this, it is sometimes difficult to keep excellent transmission quality only by setting the scrambling speed within a range of speed sufficient to average degradation of the transmission quality within one processing unit for error correction code.
In an optical communication system having a transmission path configuration more complicated than the point-to-point transmission line as discussed above, it is necessary to set an optimum polarization scrambling speed to each path set for each wavelength. However, the above known techniques do not disclose such constitution.
Meanwhile, Patent Document 3 discloses a scrambling speed of a polarization scrambler according to the type of modulation scheme, whereas Non-Patent Document 1 discloses a scrambling speed according to the value of PMD. However, these documents do not mention a technique for setting the scrambling speed in a repeater such as an OADM device accommodating a plurality of paths set for respective wavelengths.
In an optical communication system having an optical add/drop multiplexer or a transmission path configuration more complicated than a point-to-point transmission path, the modulation scheme and modulation speed of a signal light passing through a specific polarization scrambler on the transmission path and a value of PMD passing through the polarization scrambler fluctuate when the path is switched, for example. In such case, a constitution that can perform optimally polarization scrambling even when these fluctuations occur is required in order to secure excellent transmission quality.